Process for producing pure crystalline silicon by pyrolysis



De@ 3, 1963 w. c. al-:Nzms ErAL 3,112,997

mocassmmowcmcmmswm-smoousvmmms Original. F1106 00L 1. 1958 FIG.

United States Patent ce 3,112,997 Patented Dec. 3,

3,112,997 PROCESS FOR PRODUCING PURE CRYSIALLDXE SILICON BY PYROLYSIS Walter C. Bening, Momxtainside, and Edward Christoplier Schau-schmidt, Cranford, NJ., and Robert E. Gnmmer, Sunbury, Pa., assguorsv to Merck & Co.,

Ine., Rahway, NJ., n of New .l'ersey Urignal application l, 1958, Ser. No. 764,646, now Patent N0. 3,014,791, dated Dec. 26, 1961. Divided and this application Nov. 1, 1961, Ser. No. 149,246

1 Claim. (Cl. 23-223.5)

This invention relates to apparatus for the pyroiysis of silmcs, and has for its object the provision of an improved apparatus for the formation of elemental hyperpure crystalline silicon.

This application is a divisional of copending application Serial No. 764,646. tiled Omober l, 1958, now U.S. Patent No. 3,014,791 issued December 26, 1961.

As is already known., silicon can be prepared in a rel` atively pure state by various procedures. The silicon product will, however, contain traces of metal contaminants. For the most sensitive electronic uses, silicon of higher ptn'ity is required because the presence of even trace amounts of impurities proves detrimental for the most exacting semiconductor requirements, especially in the newer electronics applicatior, such as transistors.

Among the objects of this invention is an improved apparatus for the production of hyper-pure elemental silicon by the pyrolysis of silanes. A further object is to provide an apparatus for the production of hyper-pure elemental silicon of improved electrical semiconductor quality.

The objects of this invention are accomplished in an improved apparatus for producing elemental sicon which comprises an enclosed and gas tight reactor chamber which may be operated under various pressures having means for introducing silane gas and removing the liberated hydrogento maintain a desired operating pressure withinthechambenandmeansforheatingsilanestoits and depositing the sicon.

The-gas confining exrior reactor chamber ofthe apparatus is advantageously formed of a cylindrical elongatedvmelmoimdnanuprightpositionhavingremovably cled ends to provide an enclosed heated space forapyrdyssmrlt'mamtcdtherein. Thechamberis provided with-duct means for the supply ofsilanes, duct meansfortheremovalofhydrogenorother gas, and heating means. The pyrolysis unit comprises a'heatcd pyrolysis element, preferably having exterior protrusions fortlredepositionofsilicon,disposedwithiutixecham` sformed quartz1 fox-example, a quartztube, The qimrtz tube-may be ofanysutable size oei-shape,

FlGJ'saverticals'ectionz-ilviewofvanmangemcnt Aofapparatxsembodying andl Fl.2isafragmentaryenlardvcrticalsectional cylxndriml and closed at thelowerrenrl,

The apparatus illustrated in FIG. l of the drawings comprises a reactor chamber 1 consisting of an upright cylindrical vessel 2 having anges 3 and 4 at the top and bottom respectively, and an outlet connection 5. At the bottom, a closure head 6 having a flanged rim 7 is connected by a metal tlange coupler 8 to the flange 4 and the connection is sealed with a Tetlon gasket 10. At thc top the closure head ll having a anged rim 12 is connected by a metal llange coupler 13 to the ilange 3, and the connection is also scaled with the Teon gasket 14. The couplers 8 and 13 may be aluminum split bands with bolt connections (not shown) to draw them tightly around the flanges and press the flanges into sealed contact with the gaskets 10 and 14. The cylindrical part 2 and the closure heads 6 and 11 may be made of any suitable material that will withstand the operating temperature of about 400 C. and be inert to the contained material. Por operations of laboratory or pilot types these members may be made of borosilicate glass.

The bottom closure head (when made of glass) has a ball and socket joint 15 for the connection thereto oi the supply duct 16, a glass tube, to introduce the silane gas into the reactor chamber. The outlet 5 is also provided with a ball joint 17 for the removal of hydrogen from the reactor chamber through duct 18. This duct may be connected to a vacuum pump.

.Whentheclosurchead11ismadeofglass,thentral portion has a tapered ground joint 20 for receiving the ground collar 21 of the tube 22 in sealed connection. This tube is preferably formed of fused quartz which is closed at the bottom and has an interior heating element 23 formed of Nichrome wire supported near the bottom. Aquarzcapllarywbezdisinsertedintothetube and held in its supporting position by the packing plug 25. The element 23 is connected to one electrical lead 25 which passes through the quartz tube 24 and to the other electrical lead 25 which pases upward on the outsideofthetubeZ-i. 'lhiselecn'icalcircuitisconnected to any desired source of power, for example, to a power input of about 600 volt-amperes, to heat the coil 13 to about 1000" C.

The quartz tube 22has two protruding integral quartz lugs 27 and .28 fused thereon which serve m hangers for the wires'Z'l' and 28' which are hooked over the integral brackets or lugs 30 and 3l on the quartz pyrolyss 'element 32 to hold it in its suspended position surrounding, and preferably concentric with, the tube 22. The element 32I is heated toits operating temperature of 600 to800 C., mainlyhy'theradiantenergyomtheelement 23. I1 order to facilitate the deposition and growth of dense crystalline silicon, the exterior of the pyrolysis element 32 has a multiplicity of projections 33 arranged, say, apart circumferentially and spaced about 15 inches apart longitudinally on the exterior, which serve as nuclei for the starting and growth of the depositions.

Inordertofacilit'ateabettercontmlovertheheating of element 32,- the reactor chamber 2 has a surrounding coilofNichromcwireM'WPPdSaLatlinch intervals and supplied, for example, with 300'voltam qperesofpower. 'lheheatingwreiseoveredwithan Incarryingoutanoperationinanapparatushavhlgthesedimm .siomthepyrolysiselementwasheatedtoamperntme oiaboutSOOCandthechamberwascvacuatedand sweptfreeofreactivegaswithhydrogemmaintahred atltoaunosphaicpressm'eandaninertgassuchas.

viewpfamoefnmaf'pyruysaimnanemvenm helmargonurniuogminmgaamed. samen is passed into the chamber through tube 16. the slartof the operation, the heating element 23 was set with its lower end about 6 inches from the lower head clomreandwaslowe'redaboutZinchesatereach 8 moles (256 gms.) of silane have passed into the chamber. This lowerhtg of the element was done by sliding theglasmbeMthroughthestopperZS. Theoperation was stopped when 32 moles were passed into the chamber. Shane-hydrogen mixtures of varying proportions for example, up to 60% silane concentration have been pyrolyzed. TheAtemperann-e maybe measured by Chrmnel-Alumel thermooouples placed inside the reactor chamber.

Polycrystalsofdensesiliconformonthequartzprotrusions 33 and grow to a large cylindrical deposit on the pyrolysis element 32. For example, the deposits formed on the aforementioned apparatus have been 7 inches long, 2% inches in diameter weighing about 450 grams. At the end of the nm the closure 6 is removed and the pyrolysis element with its deposit of polycrystallinesiliconisremovedfromthereactorchamber. The siliconisseparatedbyshatteringthereactorelement.

FIG. 2 illustrates a modiiication of the apparatus of FIG.1onanenlargedscale. Thetube22andthepyrolysiselement32areasshownnFlG. 1. Thetube 35passthroughafx'ictionplugsuchas25ofFIG. 1 andhasoneofthepowerleads31ontheinside and theotherontheootside. 'l'hesewiresareconnected nserieswiththeatNichmmewiredwhichshelically woundarotmdtheoutsideofthetube. Thebottom ofthembeisopentopermitthewire3'ltopassaround thelowerendandconnectwiththelowerendofthe sihn.

ExampleZ Amixtnreofsilaandhelinmwaspaedthrough the apparatus described in FIG. l and FIG. 2 at the stated operating temperatures. A 191 gm. deposit of amorphous powder silicon was formed on the reactor wall and 163.5 gm. crystalline sicon was formed on the central heating element. This represents a 92% yield of silicon of which 41.7% is crystalline yield and 50.3% is amorphous powder yield.

Example 3 A mixture of silanes and hydrogen was passed simultancously (in parallel) through three apparatus as described in FIGS. l and 2 at the stated operating temperatures. Deposits of 486 gm. of crystalline silicon and 126 gm. of amorphous powder silicon were obtained in the first reactor. Deposits of 470 gms. of crystalline silicon and 83 gm. of amorphous powder silicon were obtained in the secondpunit. 'Deposits of 475 gms. of crystalline and 141 gms. ofamorphoussiliconinthethirdunitwere realized. 111e total yield ofcrystalline silicon was 65.5% and the amorphous silicon yield was 16%. Total 81.5%.

Various changes and modifications may be made in the present invention, certain preferred embodiments of whicharelxereindisclosedwithoutdepartingfromthe scope thereof;tothcextentthatthesechangesandmodicationsarewithinthescopeoftheappendedclaimthey aretobeconsideredapartofthisinvention.

What is claimed is:

In the process for producing elemental hyper-pure erystallinesiliconbythethermalonofsilane gas in a pyrolyssapparatus, the improvement which consistsof depositingthesiliconontheexteriorofaheated quartz tube, the exterior sm'face of which has a multiplicity of small quartz protrusions relatively widely spaced omthesurfaoeservingasnucleiorthestartingand growth of crystalline silicon.

References Cited in the tile of this patent UNITED STATES PATENTS 

